This invention relates to thermally stable polyvinyl halide resins. Specifically, this invention relates to polyvinyl halide resins for example polyvinyl chloride that is free of allylic chlorine and has superior heat stability than has heretofore been disclosed.
Polyvinyl halide resins are used in a variety of applications. These thermoplastic polymers can be fabricated into useful articles by extrusion, injection molding, compression molding and other thermoforming methods. Generally these methods involve mixing the resin with processing aids, heating the composition to a temperature to fuse the resin particles, forming the composition into the desired shape, and then cooling the composition to a solid. Due to the presence of unsaturation and labile allylic halogen in the polymer backbone, the resin is sensitive to dehydrohalogenation when exposed to heat. In particular, polyvinyl chloride resins are so sensitive to heat, they do not exhibit a distinct melting point and must be combined with fusing agents to facilitate their fusing during forming. At the melt temperatures, the polymer degrades and turns black. The polymer degrades because the allylic chlorines are activated when exposed to heat and are released from the polymeric backbone. A free radical remains that reacts with another part of the backbone causing crosslinking, or propagation of double bonds. Also, free chlorines react together to form Cl.sub.2 or with released hydrogen to form HCl. Although polyvinyl chloride resins are initially white, thermoformed articles that are damaged by heat during the process can range in color from yellow to black.
To stabilize polyvinyl halide resins during thermoforming methods, heat or thermal stabilizers are added to the resins. By using these stabilizers, the resin can be fused with a reduced degree of degradation and discoloration. Examples of such heat stabilizers are organometallic compounds such as metallic, alkali metal and alkaline earth metal salts of fatty acids. The metals include lead, barium, cadmium, tin, calcium, and zinc. A popular organometallic stabilizer is dibutyltin di-2-ethylhexanoate. It is believed that the stabilizers function by reacting with the labile allylic chlorines. The chlorine complexes with the metal and the organic group substitutes for the chlorine on the backbone.
Unfortunately, the stabilizers provide only relatively short-term stability. The organic group can be further eliminated when heated by the same mechanism as the chlorine is eliminated as in PVC. Moreover, the stabilizers can be hazardous to the environment or even toxic, can themselves discolor the resin, and can also be incompatible with the resins. Since the metal remains mixed with the resin as a metal chloride complex, it can leach out of, or migrate from the resin or formed article. This can cause cracking in the molded article as well as cause damage to other articles that are in contact with the resin-formed article. Also, in stabilized PVC this residual metal chloride can contribute to degradation of the polymer at high processing temperatures. The metal chloride complex contributes to crosslinking and viscosity increase of the polymer at high temperatures. This viscosity increase is observed when the compound is processed at 200.degree. C. for about 25 minutes. The amount of torque required to process the compound increases dramatically as the viscosity increases. Thus, processing costs are increased. Furthermore, the substitution of the organo group on the backbone alters the structure of the polyvinyl chloride resin and can affect its properties. Thus, although the stabilizers are effective in a certain range of temperatures for stabilizing polyvinyl chloride resins during thermoforming processing, they have disadvantages that discourage their use.
Conventional alkyl tin stabilizer additives known to displace allylic chlorines substitute therefore alkyl groups at the allylic carbon. This treatment can be carried out by treating the resin with a stabilizing treatment solution. In this method, the resin is dissolved in a solvent, and the treatment solution is then added. The resin is then precipitated from the solution and recovered. Examples of treatment solutions are potassium allyl xanthate in dimethyl formamide, di(n-butyl)tin bis (n-dodecyl mercaptide) or mixtures with di(butyl)tin dichloride in o-dichloro-benzene, and dialkylaluminum chloride with a lower alkanol. Again, these compounds substitute the labile allylic chlorines and introduce a foreign structure onto the backbone other than hydrogen. For example, in the dialkylaluminum chloride and lower alkanol treatment, the dialkylaluminum chloride is sued to catalyze the substitution of the organic portion of the alkanol to the polymer backbone. Thus, the organic portion is attached to the backbone through an ether-linkage. This results in what is known as allylic branching. This allylic branching is nevertheless a more labile site as compared to the --CH.sub.2 -- structure absent an alkyl branch.
Therefore, considering the many uses of polyvinyl halide articles, resins that have superior heat stability are desired. Also, in view of the disadvantages associated with the use of stabilizers and organic treatments, it would be desirable to produce a vinyl halide polymer, for example a vinyl chloride polymer which is absent allylic chlorine and allylic branching as analyzed by the precise .sup.1 H proton Nuclear magnetic resonance spectroscopic method. Furthermore, it would also be highly desirable to provide a vinyl chloride polymer absent allylic chlorine, allylic branching and unsaturation.